The present invention relates to a cathode active material for a non-aqueous electrolyte secondary cell and a process for producing the active material, and more particularly, to a cathode active material for a non-aqueous electrolyte secondary cell which is capable of maintaining an initial discharge capacity required for secondary cells and showing improved charge/discharge cycle characteristics under high temperature conditions, and a process for producing such a cathode active material.
With the recent rapid development of portable and cordless electronic devices such as audio-visual (AV) devices and personal computers, it has been increasingly demanded to use as a power source thereof, secondary cells or batteries having a small size, a light weight and a high energy density. Under this circumstance, among the secondary cells, lithium ion secondary cells have been noticed because of advantages such as high charge/discharge voltages as well as high charge/discharge capacities.
Hitherto, as cathode active materials useful for high energy-type lithium secondary cells exhibiting a 4V-grade voltage, there are generally known LiMn2O4 having a spinel structure, LiMnO2 having a corrugated layer structure, LiCoO2 having a layered rock salt-type structure, LiCo1xe2x88x92xNixO2, LiNio2 or the like. Among secondary cells using these active materials, lithium ion secondary cells using LiCoO2 are more excellent because of high charge/discharge voltages and high charge/discharge capacities. These lithium ion secondary cells have been, however, required to be further improved in properties thereof.
Specifically, when the lithium ion secondary cell using LiCoO2 as an active material is repeatedly subjected to charge/discharge cycles, the discharge capacity of the secondary cell tends to be deteriorated. This is because LiCoO2 undergoes Jahn-Teller distortion due to conversion of Co3+ of LiCoO2 into Co4+ when lithium ions are released therefrom. As the amount of lithium released increases, the crystal structure of the active material is transformed from hexagonal system into monoclinic system, and further from monoclinic system into hexagonal system. In addition, when such release and insertion reactions of lithium ions are repeated, the lattice of LiCoO2 suffers from contraction and expansion, resulting in destruction of the crystal structure of LiCoO2. As a result, it is assumed that the charge/discharge cycle characteristics of the secondary cell are deteriorated.
Also, when the secondary cell is repeatedly subjected to charge/discharge reactions (i.e., release and insertion reactions of lithium ions), the crystal structure of the active material becomes unstable, thereby causing release of oxygen from the crystal lattice, or undesired reaction with an electrolyte solution. Further, the reaction with an electrolyte solution is more active under high temperature conditions. Therefore, in order to ensure safety of the secondary cell, it has been required to provide active materials exhibiting a stable structure and a high heat stability even under high temperature conditions.
Since electronic devices using the secondary cell as a power source, such as note-type personal computers, are exposed to high temperature due to heat generation upon use, it has been required to provide secondary cells showing excellent charge/discharge cycle characteristics even under high temperature conditions.
Also, the secondary cell using LiCoO2 can be operated with a high voltage. However, since the LiCoO2 is readily reacted with an electrolyte solution under a high-voltage condition, the secondary cell tends to be deteriorated in charge/discharge cycle characteristics.
For this reason, it has been required to provide lithium cobaltate particles which are capable of producing secondary cells exhibiting excellent charge/discharge cycle characteristics even under high temperature conditions.
Hitherto, in order to improve various properties, e.g., in order to stabilize a crystal structure thereof, there are known a method of incorporating manganese into lithium cobaltate particles (Japanese Patent Publication (KOKOKU) No. 7-32017 (1995) and Japanese Patent Application Laid-Open (KOKAI) No. 4-28162 (1992)); a method of incorporating magnesium into lithium cobaltate particles (Japanese Patent Application Laid-Open (KOKAI) Nos. 6-168722 (1994), 11-102704 (1999), 2000-11993 and 2000-123834); a method of mixing manganese or magnesium with lithium cobaltate particles by a wet method (Japanese Patent Application Laid-Open (KOKAI) Nos. 10-1316 (1998) and 11-67205 (1999)); a method of controlling a lattice constant of lithium cobaltate to improve properties thereof (Japanese Patent Application Laid-Open (KOKAI) No. 6-181062 (1994)); or the like.
At present, it has been required to provide cathode active materials satisfying the above requirements. However, such cathode active materials have not been obtained until now.
That is, in Japanese Patent Application Laid-Open (KOKAI) Nos. 7-32017 (1995), 4-28162 (1992), 6-168722 (1994), 11-102704 (1999), 2000-11993 and 2000-123834, it is described that a cobalt compound, a lithium compound and manganese or magnesium are dry-mixed with each other, thereby obtaining lithium cobaltate particles containing manganese or magnesium. However, since the obtained lithium cobaltate particles have a non-uniform distribution of manganese or magnesium, the crystal structure thereof tends to undergo contraction and expansion upon the release and insertion reactions of lithium ions, resulting in destruction of the crystal lattice. Thus, the secondary cells using such lithium cobaltate particles fail to show excellent charge/discharge cycle characteristics under high temperature conditions.
In Japanese Patent Application Laid-Open (KOKAI) No. 10-1316 (1998), there is described the process for producing lithium cobaltate particles by dispersing a cobalt compound and manganese compound or magnesium compound in an aqueous lithium hydroxide solution and heat-treating the resultant dispersion. However, this process requires an additional hydrothermal treatment and, therefore, is industrially disadvantageous.
In Japanese Patent Application Laid-Open (KOKAI) No. 11-67205 (1999), there is described the process for producing lithium cobaltate particles by mixing a solution containing water-soluble salts of lithium, cobalt and manganese with a citric acid solution, gelling the resultant mixed solution by removing a solvent therefrom, and drying and sintering the obtained gel. However, the obtained lithium cobaltate particles have a large BET specific surface area and, therefore, undesirably show a high reactivity with an electrolyte solution when used in secondary cells.
Also, in Japanese Patent Application Laid-Open (KOKAI) No. 6-181062 (1994), there is described lithium cobaltate having a c-axis length of lattice constant of not less than 14.05 xc3x85. However, secondary cells using the above lithium cobaltate cannot be sufficiently improved in charge/discharge cycle characteristics under high temperature conditions as compared to those using lithium cobaltate particles containing manganese or magnesium.
As a result of the present inventors"" earnest studies for solving the above problems, it has been found that by adding an aqueous alkali solution to a solution containing a cobalt salt and a manganese salt with or without a manganese salt to conduct a neutralization reaction therebetween; oxidizing the resultant solution to obtain a cobalt oxide containing manganese or both manganese and magnesium; mixing the thus obtained cobalt oxide with a lithium compound; and heat-treating the resultant mixture, the obtained lithium cobaltate particles are useful as a cathode active material for a non-aqueous secondary cell not only having an excellent initial discharge capacity, but also exhibiting excellent charge/discharge cycle characteristics under high temperature conditions. The present invention has been attained based on the finding.
It is an object of the present invention to provide lithium cobaltate particles which are useful as a cathode active material for a non-aqueous secondary cell not only having an excellent initial discharge capacity but also exhibiting excellent charge/discharge cycle characteristics under high temperature conditions.
It is another object of the present invention to provide a process for producing the lithium cobaltate particles which are useful as a cathode active material for a non-aqueous secondary cell not only having an excellent initial discharge capacity required for secondary cells, but also exhibiting excellent charge/discharge cycle characteristics under high temperature conditions.
To accomplish the aim, in a first aspect of the present invention, there is provided a cathode active material for a non-aqueous electrolyte secondary cell, having a c-axis length of lattice constant of 14.080 to 14.160 xc3x85, an average particle size of 0.1 to 5.0 xcexcm, and a composition represented by the formula:
LiCo(1xe2x88x92xxe2x88x92y)MnxMgyO2
wherein x is a number of 0.008 to 0.18; and y is a number of 0 to 0.18.
In a second aspect of the present invention, there is provided a process for producing the cathode active material for a non-aqueous electrolyte secondary cell, comprising:
adding an aqueous alkali solution to a solution containing a cobalt salt and a manganese salt with or without a magnesium salt to conduct a neutralization reaction therebetween;
oxidizing a resultant mixed solution by passing an oxygen-containing gas therethrough to obtain a cobalt oxide containing manganese or both manganese and magnesium;
mixing the cobalt oxide with a lithium compound; and
heat-treating a resultant mixture of the cobalt oxide and the lithium compound.
In a third aspect of the present invention, there is provided a cathode active material for a non-aqueous electrolyte secondary cell, having a c-axis length of lattice constant of 14.080 to 14.160 xc3x85, an average particle size of 0.1 to 5.0 xcexcm, and a composition represented by the formula:
LiCo(1xe2x88x92xxe2x88x92y)MnxMgyO2
wherein x is a number of 0.008 to 0.18; and y is a number of 0 to 0.18,
produced by a process comprising:
adding an aqueous alkali solution to a solution containing a cobalt salt and a manganese salt with or without a magnesium salt to conduct a neutralization reaction therebetween;
oxidizing a resultant mixed solution by passing an oxygen-containing gas therethrough to obtain a cobalt oxide containing manganese or both manganese and magnesium;
mixing the cobalt oxide with a lithium compound; and
heat-treating a resultant mixture of the cobalt oxide and the lithium compound.
In a fourth aspect of the present invention, there is provided a cathode active material for a non-aqueous electrolyte secondary cell, having a c-axis length of lattice constant of 14.080 to 14.160 xc3x85, an a-axis length of lattice constant of 2.81 to 2.83 xc3x85, a crystallite size of 400 to 1,200 xc3x85, an average particle size of 0.1 to 5.0 xcexcm, and a composition represented by the formula:
LiCo(1xe2x88x92xxe2x88x92y)MnxMgyO2
wherein x is a number of 0.008 to 0.18; and y is a number of 0 to 0.18.
In a fifth aspect of the present invention, there is provided a cathode active material for a non-aqueous electrolyte secondary cell, having a c-axis length of lattice constant of 14.080 to 14.160 xc3x85, an a-axis length of lattice constant of 2.81 to 2.83 xc3x85, a crystallite size of 400 to 1,200 xc3x85, an average particle size of 0.1 to 5.0 xcexcm, and a composition represented by the formula:
LiCo(1xe2x88x92xxe2x88x92y)MnxMgyO2
wherein x is a number of 0.008 to 0.18; and y is a number of 0.01 to 0.15.
In a sixth aspect of the present invention, there is provided a non-aqueous electrolyte secondary cell comprising a lithium ion conductive electrolyte and a pair of electrodes separated by means of a separator, wherein at least one of said electrodes comprises a cathode active material having a c-axis length of lattice constant of 14.080 to 14.160 xc3x85, an average particle size of 0.1 to 5.0 xcexcm, and a composition represented by the formula:
LiCo(1xe2x88x92xxe2x88x92y)MnxMgyO2
wherein x is a number of 0.008 to 0.18; and y is a number of 0 to 0.18.